1. Field of the Invention
The present invention relates to a vehicle driveline clutch and, more particularly, to a driveline clutch where a friction disc is clamped to an engine flywheel using a ball ramp actuator wherein an electromagnetically actuated cone clutch is used to control the torque transferred to the ball ramp actuator.
2. Description of the Prior Art
Driveline master clutches commonly use a plurality of springs to clamp a friction disc to an engine flywheel. The springs are disposed within a pressure plate assembly which is bolted to the flywheel. A mechanical linkage that controls the pressure plate spring mechanism is displaced by the operator to control the lock-up and release of the driveline clutch.
Efforts to automate the operation of the driveline clutch using electronics are currently underway. It is known to use an electromechanical or hydraulic actuator connected to the mechanical linkage to, in essence, replace the operator for more accurate clutch operation during transmission shifting. Using such an actuator, the mechanical linkage is moved in response to an electrical control signal generated by a central microprocessor used to process a variety of vehicle sensor inputs based on operating conditions to determine when and in what manner the driveline clutch should be activated or deactivated.
The use of a ball ramp actuator to load a clutch pack in a vehicle driveline differential is known. U.S. Pat. Nos. 4,805,486 and 5,092,825, the disclosures of which are hereby incorporated by reference, disclose limited slip differentials where a clutch pack is loaded in response to the activation of a ball ramp actuator initiated by rotation of a servo motor or a solenoid driven brake shoe on an activation ring. The advantage of the ball ramp mechanism over other actuators is that it converts rotary motion into axial motion with very high force amplification, often 100:1 or greater. A ball ramp actuator has also been utilized in a vehicle transmission to engage and disengage gearsets by loading a gear clutch pack in response to a signal as disclosed in U.S. Pat. No. 5,078,249, the disclosure of which is hereby incorporated by reference.
In both of these vehicle applications, one side of the ball ramp actuator, commonly called a control ring, reacts against case ground through the force induced by an electromagnetic field generated by a coil or is rotated by an electric motor relative to case ground. To generate greater clamping forces, the electrical current supplied to the coil or motor is increased thereby increasing the reaction of the control ring to case ground which rotates the control ring relative to an actuation ring thereby causing rolling elements to traverse ramps in the control and actuation ring which increase the axial movement and clamping force on the clutch pack.
It is also known to use a ball ramp actuator to load a vehicle driveline clutch as disclosed in U.S. Pat. Nos. 1,974,390; 2,861,225; 3,000,479; 5,441,137; 5,469,948; 5,485,904 and 5,505,285, the disclosures of which are hereby incorporated by reference. One problem with the use of a ball ramp actuator to supply the vehicle driveline clutch clamping force is that the mechanics of prior art unidirectional ball ramp mechanisms result in a loss of clamping force when the vehicle is in a coast mode. Once the engine power is reduced and the driveline is actually overrunning the engine (coast mode), the prior art ball ramp actuator with single ramp unidirectional actuation will disengage the clutch thereby eliminating engine braking of the vehicle.
A ball ramp actuated clutch using a unidirectional ball ramp having only a single ramp angle, will cause the clutch to disengage when the engine is not supplying rotational energy into the transmission such as when the vehicle is coasting. When coasting, the flywheel is no longer supplying rotational energy to either the transmission or to the ball ramp actuator. In this circumstance, the relative rotation of the actuation ring and control ring has been reversed such that the ball ramp axial displacement is collapsed thereby allowing the pressure plate to pull away from the clutch disc. The result is that the engine is disengaged from the transmission and any engine braking effect is eliminated.
A bidirectional ball ramp actuated clutch is disclosed in U.S. Pat. Nos. 2,937,729 and 5,505,285. Using this more expensive and complicated technology, the ball ramp actuator incorporates bidirectional ramps which provide activation when there is relative rotation between the control ring and the actuation ring in either direction. However, the ball ramp must transition through the nonactivated state which will result in temporary undesirable clutch slippage and, in addition, the components are more expensive to fabricate than a unidirectional unit. Also, a bidirectional ball ramp will have reduced rotational travel between the control ring and the actuation ring in a given package size as compared to a unidirectional ball ramp mechanism. Thus, a unidirectional ball ramp mechanism is preferred if it can be made to activate in both vehicle drive and coast operating modes.
The ball ramp actuator comprises a plurality of rolling elements, a control ring and an opposed actuation ring where the actuation ring and the control ring define at least three opposed single ramp surfaces formed as circumferential semi-circular grooves, each pair of opposed grooves containing one rolling element. A thrust bearing is interposed between the control ring and a housing member, rotating with and connected to the input member such as a flywheel. An electromagnetic coil is disposed adjacent to one element of a control clutch so as to induce a magnetic field that loads the control clutch which in turn applies a force on the control ring of the ball ramp actuator. The control clutch can be similar to those commonly used for vehicle air conditioning compressors, or a cone type control clutch to increase the transmitted actuation force. In an electromagnetic type of clutch, the air gap present in the clutch is traversed by an armature when the coil is energized and the forces on the armature increase in an exponential manner as the armature is drawn toward the stator/coil. The dramatic force increase as the armature is moved results in difficulty in controlling the operation of the ball ramp mechanism and hence the driveline clutch which is loaded by the ball ramp mechanism. Thus, it would be desirable to eliminate or minimize the effect of the air gap in the control clutch for smoother engagement of the driveline clutch especially during vehicle start up.